The field of this invention, generally is heat exchangers and, more particularly, it deals with the type of condensing and evaporating system referred to in the art as a heat pipe.
While water is a highly desirable heat pipe fluid for operating temperatures between 50.degree. C. and 250.degree. C. because of its high latent heat or vaporization, a severe limitation exists in the potential threat of damage to a water loaded heat pipe, due to freezing of the water.
When a water heat pipe freezes, the expansion resulting as the water changes to ice can cause rupture of the heat pipe casing in much the same way as household plumbing is damaged by freezing. However, a second mode of failure can also be brought on by freezing. If the water is located within the rigid wick structure adjacent to the casing at the time of freezing, the expansion can cause distortion in the wick structure. This distortion moves the wick away from the casing wall and thereby prevents both evaporation and condensation from that portion of the wick. Such damage is irreversible and permanently degrades the performance of the heat pipe proportional to the amount of wick which is no longer in intimate contact with the casing.
The freezing problem is particularly serious if a heat pipe freezes when in a vertical or in an inclined position rather than in the horizontal position. In such situations a puddle of water which spans the entire diameter can form at the lower end of the heat pipe, and such a puddle, when frozen, exerts considerable force on the heat pipe wick and casing, frequently causing rupture of the casing or distortion of the wick.
The only approach to solving this problem to date has been the most obvious one, preventing freezing of the liquid. However, in commercial, as opposed to laboratory, operations such precautions are not always feasible, and the actual result has been a reluctance to use freezing prone liquids, such as water, in heat pipes.